Documentation improvement #44

You could add somewhere on README that if virtualenv is used, it must be created using --system-site-packages otherwise python doesn't have access to PyQt4 lib and error is shown.

virtualenv env --system-site-packages

Spent lot of time wondering why even after installing all the dependencies i got error when running webkit2png

>>> from PyQt4.QtCore import *
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named PyQt4.QtCore